Bayan Obo is a stratabound ore deposit hosted by Proterozoic marble of the Bayan Obo Group. The regional distributions of the Proterozoic Bayan Obo Group, the Archean metamorphic rocks of the Wutai Group, which unconformably underlies the Bayan Obo Group, and granitic rocks of Archean, Caledonian (early to late Paleozoic), and Hercynian (Permian) ages in the region are shown in figure 1. Alkaline plutonic or volcanic rocks have not been reported in this region and evidently are not present. The Bayan Obo mine area (fig. 2) is separated from the area to the north by a major fault zone, the Kuanggou fault. The differences of lithology and degree of metamorphism and mineralization on either side of the fault indicate that the age of the Kuanggou fault is probably post-late Caledonian (younger than 400 Ma; Chao and others, 1992). The geology of the region north of the Kuanggou fault, which is broken by many extensional and thrust faults, is distinctly different from that of the mine area, and the stratigraphic sequence there has been designated as an undifferentiated unit of Proterozoic to Paleozoic sedimentary rocks.
Figure 1. Geologic sketch map of the Bayan Obo region (modified from Institute of Geochemistry, 1988).
Figure 2. Generalized geologic map of the Bayan Obo study area (modified from Chao and others, 1992).
According to Bai and Yuan (1985), Li Yuying subdivided the Bayan Obo Group into nine lithologic units; a detailed stratigraphic section measured in the vicinity of the mine by Meng (1982) also distinguished nine lithologic units. This section was subsequently described in greater detail by Chao and others (1992). The lower part of the group (Hu, fig. 2) consists of arkosic quartzite (H1), fine- to medium-grained quartzite (H2), dark carbonaceous slate (H3), arkosic quartzite and slate (H4), slate with intercalated silty sandstone and quartzite (H5), a ridge-forming quartzite (H6), and transitional beds of dolostone marble, arkosic sandstone, and calcareous slate (H7). Overlying this lower sequence is the dominant ore-bearing unit, H8, which ranges from 240 to 540 m in thickness, the thickest part being near the Main and the East Orebodies. The H8 unit consists of dolostone marble and subordinate limestone marble, intercalated with quartzite in the upper and lower parts. Small amounts of porphyroblastic albite biotite schist and biotite-rich minette occur as lenses within this unit. The uppermost lithologic unit, H9, 340-350 m thick, consists of a sequence of interbedded gray to black shale, slate, and biotite schist that conformably overlies the H8 unit.
A small swarm of 11 calcitic carbonatite dikes, mostly 1-2 m wide and up to 150 m long, cut across an anticlinal structure in the Archean Wutai granitic gneisses and migmatites, approximately 1.5 km northeast of the mine area. The largest of these dikes was studied in detail by Le Bas and others (1992). Another meter-wide, dolomitic carbonate dike that cuts H1 or H4 arkosic quartzite northeast of the mine was recently discovered. The alkali amphibole in the fenitized contact zone of this carbonatite gave an 40Ar/39Ar plateau age of 1.28 Ga (Ren Yingchen, oral commun., 1993).
South of the Kuanggou fault zone, the Bayan Obo Group crops out in a broad syncline in which the H8 and H9 units form the central part. The north limb of the syncline dips generally 50°-70° S. The south limb is steeper, generally dipping 70°-80° N., but locally it is vertical or is slightly overturned (Meng, 1982). This syncline has many small chevron folds, meter-wide shear zones, and thrust faults, all trending east-west. Many of the more strongly mineralized zones in the orebodies follow the east-trending shear zones.
On the basis of sequential stratigraphic analysis, Qiao and others (1991) correlated the Bayan Obo Group with the Chartai Group and interpreted both groups as having been deposited on the North China Platform or craton (fig. 3). The Chartai Group was deposited on the continental shelf, whereas the Bayan Obo Group was deposited on the continental slope of the craton. The Bayan Obo mine area is about 100 km south of the suture marking Caledonian subduction (fig. 4; Wang, 1985) that took place at the northern edge of the North China Platform or craton. A small granitic intrusion of Caledonian age occurs in the Hejao area, south of Bayan Obo (fig. 1). No direct evidence of rifting involving the Bayan Obo Group in the mine region was observed, although rifting was cited by Wang and others (1991) on the basis of geophysical evidence and the reported occurrence of volcanic rocks in the Chartai and Bayan Obo Groups.
Figure 3. Deposit relation of the Chartai and Bayan Obo Groups as cover beds deposited on the continental slope of the North China Platform or craton (modified from Qiao and others, 1991). A, Advance of the sea at an earlier stage, when the Chartai Group was deposited. B, Retreat of the sea at a later stage, when the Bayan Obo Group was deposited.
Figure 4. Paleotectonic map of northern China in the Caledonian period (modified from Wang, 1985), showing locations of the Caledonian subduction, the Hercynian subduction, and the Caledonian Hejao anorogenic granites.
Field studies consisted of identifying and sampling the many rock and ore types in the Bayan Obo mine area and determining the geographical extent and relations of the ore types. About 830 samples, including samples from three drill cores, were collected in the summers of 1987, 1988, and 1990. Sampling localities in and near the Main and East Orebodies are shown in figure 5. Locations of other samples provided by colleagues of the Tianjin Geological Research Academy, which are not known, are not given in figure 5. A few samples from the West Orebodies (16 small orebodies in an area extending about 8 km west of the Main Orebody) are also not shown in figure 5. In the summers of 1988 and 1990, the preliminary mineral paragenetic sequences established petrographically from doubly polished thin sections (DPTS's) were checked in the field to confirm consistencies with field relations. Samples collected in 1987, 1988, and 1990 are numbered with a prefix 7B, 8B, and 9B, respectively, followed by the sample location number. For example, samples from 7B30 were collected in 1987 at sample location number 30. Samples 7B30-1, 7B30-20, and 7B30-30 are three separate samples from sample location 7B30. Except for drill cores, samples without a prefix 7B, 8B, or 9B were provided by colleagues from Tianjin.
Figure 5. Localities of samples taken in and near the Main and East Orebodies of Bayan Obo.
In this paper, we use the following grain-size designations: coarse, >500 micrometers; medium, 250-500 micrometers; fine, 125-250 micrometers; very fine, 62-125 micrometers; and extremely fine, <62 micrometers. Because of the extremely fine grain size of most of the ore and gangue minerals in the Bayan Obo area, interference microscopy was used for identification of the minerals in DPTS's (Chao and Xie, 1990). Routine identification of very and extremely fine grained minerals was accomplished by preliminary optical microscopy and verified by scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDX). The optical and SEM-EDX studies were iterative because SEM-EDX usually examined smaller areas than optical microscopy. X-ray diffraction techniques were rarely used because of the difficulty of obtaining pure mineral concentrates from the extremely fine grained ores. Both optical microscopy and SEM were used for textural analysis of mineral replacement relations.
Most mineral compositions were obtained by electron microprobe analysis (EPMA), using carefully selected and calibrated standards. Other techniques were also used to determine compositions of individual minerals. For example, monazites and bastnaesites were analyzed by isotope dilution techniques (Wang and others, 1994), by instrumental neutron activation analysis (INAA), and by synchrotron X-ray fluorescence analysis (SXRF) in a few cases (Chen and others, 1993). Wavelength-dispersive X-ray fluorescence spectrometry (WDXRF) was used for analyzing unmineralized and weakly mineralized dolostone marble. Because of the high contents of combined F, Fe, and REE's, inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used for partial analysis of most of the disseminated and banded ores. BaO, SrO, Ce2O3, La2O3, Nd2O3, and Nb2O5 contents of unmineralized and REE-mineralized H8 dolostone marble were determined by inductively coupled plasma-mass spectrometry (ICP-MS). Fluorine and chlorine contents of the ores were determined by the ion-selective electrode (ISE) method (Jackson and others, 1987). CO2 content was determined by using coulometric titration (Jackson and others, 1987). The WDXRF and ICP-AES methods of analysis were described by Taggart and others (1987) and Lichte and others (1987), respectively.
Mineral separations for radiometric-isotopic analysis and age determination were made in the laboratories of the Tianjin Geological Research Academy, in Tianjin, and in USGS laboratories in Reston, Va. K-Ar and 40Ar/39Ar ages were determined in USGS laboratories in Menlo Park, Calif. Precise internal isochron mineral ages of monazites and bastnaesites were determined by the 232Th/208Pb method in USGS laboratories in Denver, Colo.
The dominant host rock replaced by the Bayan Obo ores is H8 marble, followed by interbedded quartzite within the H8 unit, and overlying H9 biotite schist and albitized biotite schist. The H8 unit consists chiefly of dolostone marble but has subordinate limestone marble; interbeds of quartzite and feldspathic quartzite are present throughout the unit but are most prevalent in the upper and lower parts. Sparse layers of albitized biotite schist and biotite-rich (minette) lenses also are present. The H8 unit consists of a simple suite of rock-forming minerals (table 1), of which deformed and stretched ferroan dolomite is dominant in dolostone marble, calcite in limestone marble, and quartz in quartzite. Detrital mineral grains that are present include quartz, apatite, and albite in dolostone marble, apatite being abundant in coarse-grained dolostone marble. Detrital albite grains also occur in feldspathic quartzite, and albite porphyroblasts are found in albitized biotite schist. Scattered prismatic alkali amphiboles of metamorphic origin are present in the dolostone marble, where they are most abundant along foliation planes.
Table 1. Principal minerals of the Bayan Obo ore deposit.
The distribution, sedimentary structures and textures, and recent stable oxygen and carbon isotope data all indicate a sedimentary origin for the H8 marble unit. Furthermore, the H8 unit crops out over an area 16 km long and 2-3 km wide, an outcrop area much greater than that of any known igneous carbonatite. Although metamorphism has caused intense mylonitization and recrystallization of H8 carbonate, alternating thick and thin beds of the original sedimentary unit are still evident at many places (Chao and others, 1992). Original sedimentary features such as ripple marks, although rare because of the metamorphic effects, have been observed locally (Meng, 1982), and relict oolitic textures have been reported (Meng and Drew, 1992). Detrital quartz grains are present in the thin-bedded, more fine-grained marble (fig. 6), and rounded detrital apatite grains are present in the coarser grained marble (fig. 7). In addition, fossilized blue-green algae and fungal material have been found in the mine region (Meng, 1982). Meng and Drew (1992) reported new data on stable oxygen and carbon isotopes from six selected samples of relict micrite (very fine grained carbonate rock) from the H8 dolostone marble. One of these samples has a delta18O value of 13.9 and a d13C value of -9.27. The other five samples have delta18O values ranging from 16.5 to 21.0 and delta13C values from -2.42 to -4.92. These authors concluded that the relict micrite component of the dolostone samples is of sedimentary origin.
Figure 6. Photomicrograph of sample 7B34, a fine-grained dolostone marble, showing banding parallel to bedding and lenses of scattered detrital quartz grains.
Figure 7. Photomicrograph of sample 7B34-16, a coarse-grained dolostone marble containing rounded grains of detrital apatite and showing triple-junction grain boundary texture.
Thin beds of fine-grained to very fine grained high-purity quartzite are present in the H8 marble unit and locally contain sparse grains of detrital albite feldspar. In thin section, quartzite shows triple-junction grain boundaries; in highly sheared parts, the grains are stretched, having a width-to-length ratio of from 1:5 to more than 1:10. Thin sections also show mortar texture wherein granulated fine-grained quartz occurs between large quartz grains.
Wang and others (1990, 1992) and Liu and Wang (1990) reported the presence of small shelly fauna of Early Cambrian age (the Qinghezhen fauna) in the Bayan Obo Group both north and south of the Bayan Obo region. This finding implies that the Bayan Obo Group may not be of Precambrian age. However, Li and Zhang (1993) and Zhang (1994) refuted this finding when they reinvestigated the Qinghezhen small shelly fauna and found them to be artificially produced when dilute hydrofluoric acid (HF) was used to extract tubular structures from the carbonate rocks.
A Middle Proterozoic age was attributed to the H1-H9 Bayan Obo Group on the basis of the presence of fossilized blue-green algae and fungal material (Meng, 1982) and on the basis of sequence stratigraphic analysis (Qiao and others, 1991). The significance is questionable of the Pb-Pb and Sm-Nd model ages of 1400-1500 Ma that were used by previous investigators to estimate the age of the H8 (Chao and others, 1992; Conrad and McKee, 1992; Wang and others, 1994). The minimum age of the Bayan Obo Group is constrained by a 40Ar/39Ar age spectrum analysis of alkali amphibole, which is interpreted to reflect an age of 1.26 Ga, from a vein cutting the H6 quartzite (Conrad and McKee, 1992; see also sample Drew 1 in table 15). Alkali amphibole from another vein cutting the H6 quartzite in the western part of the mine area gave a 40Ar/39Ar plateau age of about 820 Ma (Conrad and McKee, 1992; see also sample 7B13C in table 15). A minimum age of the H8 marble also is indicated by the 40Ar/39Ar ages of alkali amphiboles occurring along foliation planes in the marble, which in spite of varying degrees of argon loss indicated by age spectrum and total fusion analyses (fig. 33), are as old as 890 Ma (samples 40-14 and 8B24-1, table 15). If the dolostone marble is similar in age to the H6 quartzite, the minimum age of H8, as well as the entire Bayan Obo Group, is about 1.26 Ga, or Middle Proterozoic. By contrast, the earliest age of REE mineralization is 555 Ma (Wang and others, 1994), more than 70 million years (m.y.) younger than the H8 unit.
On the basis of field and laboratory textural evidence, including an unpublished cathodoluminescence study by Meng Qingrun (unpub. data, 1988), the H8 carbonate rocks were probably deposited as a limestone unit, which was dolomitized after lithification by addition of large quantities of Mg and Fe to form the principal mineral, ferroan dolomite. Subsequent regional metamorphism caused mylonitization and recrystallization of this ferroan dolomite long before mineralization took place.
EPMA of ferroan dolomite from three dolostone marble samples of the H8 unit is given in table 2. Of these, sample 7B25-1C is coarse-grained dolomite marble; 7B36-10C is coarse-grained ferroan dolomite embedded in a fine-grained ferroan dolomite "matrix" (7B36-10F); and 7B34-15C is coarse-grained ferroan dolomite surrounded by fine-grained ferroan dolomite, 7B34-15F, of the same sample. The MgO and FeO contents of both the coarse-grained ferroan dolomite and the fine-grained dolomite are variable, ranging from 13.78 to 19.54 wt. percent MgO and from 3.77 to 10.47 wt. percent FeO. Textural relations indicate that the fine-grained ferroan dolomite in some of the marble was formed by mylonitization or granulation during regional metamorphism. EPMA indicates that neither the coarse- nor the fine-grained ferroan dolomite was chemically homogenized after recrystallization. It is notable that in a carbonate environment, large quantities of Fe introduced during dolomitization did not precipitate as primary oolitic Fe ore of sedimentary origin.
Table 2. Chemical composition (wt. percent) of ferroan dolomite from H8 dolostone marble of Bayan Obo.
The chemical composition of essentially unmineralized dolostone marble (table 3) is an essential parameter for evaluation of a syngenetic versus epigenetic origin of the disseminated REE ores of Bayan Obo. A study of DPTS's of unmineralized dolostone marble (samples 7B25-1 and 7B34-16) from the Bayan Obo mine area shows that the marble contains neither monazite nor bastnaesite. The two samples have essentially the same bulk chemical compositions, and there is variation in the concentration of minor components such as RE2O3 and P2O5. The total RE2O3 and P2O5 contents of sample 7B34-16 are 0.08 wt. percent (800 ppm) and 3.23 wt. percent, respectively, whereas the contents of these constituents in sample 7B25-1 are 0.04 wt. percent (400 ppm) and 1.52 wt. percent. These values are several times higher than the averages for limestones elsewhere (Bai and Yuan, 1985). Petrographically, coarse-grained dolostone marble such as that of samples 7B25-1 and 7B34-16 contains detrital apatite (fig. 7). It is probable that the small amount of REE's in unmineralized dolostone marble is contained chiefly in the detrital apatite, which possibly had its source in the Archean cratonal landmass. In contrast, sample 7B34-14, a very weakly mineralized dolostone marble that contains extremely fine grained monazite, has 0.76 wt. percent (7,600 ppm) RE2O3 and 5.81 wt. percent SiO 2. DPTS's show the presence of detrital quartz, which accounts for the high concentration of SiO2, and small elongate aggregates of fine-grained monazite, which account for the RE 2O3 present (table 3).
Table 3. Chemical composition (wt. percent) of essentially unmineralized H8 dolostone marble of Bayan Obo.
Throughout the outcrop area (fig. 2), most of the fine-grained dolostone marbles of the H8 carbonate unit show at least minor evidence of REE mineralization. In hand specimens of fine-grained dolostone marble, pale yellow streaks indicate the presence of monazite or bastnaesite. Coarse-grained dolostone marble locally also contains disseminated monazite or bastnaesite, but in general, the coarse-grained marble is less fractured, granulated, and mineralized than the fine-grained marble. Table 4 lists six samples of weakly mineralized dolostone marble in which the mineralization consists of increasing amounts of total REE's ranging from 1.05 to 4.20 wt. percent total RE2O3. Therefore, we conclude that the REE's in unmineralized Bayan Obo dolostone marble are present only in detrital apatite and that RE2O3 contents greater than about 800 ppm in fine-grained dolostone marble are due to weak epigenetic mineralization. Thus, our findings do not support Bai and Yuan's (1985) conclusion that the unusually high amount of REE's in Bayan Obo carbonate host rocks is of sedimentary-volcanogenic syngenetic origin.
Table 4. Chemical composition (wt. percent) of REE-mineralized H8 dolostone marble of Bayan Obo.
The uppermost H9 unit of the Bayan Obo Group, which is the cap rock of the Bayan Obo orebodies, consists of dark-gray to black shale, slate, biotite schist, and albitized biotite schist. It conformably overlies the H8 marble unit. The least metamorphosed parts of the H9 unit are black shales that contain detrital quartz and illite, whereas the more intensely metamorphosed H9 shales now are biotite and albitized biotite schists, which are much more dominant in the mine region than the relatively unmetamorphosed shales. Near the Bayan Obo orebodies, chevron folds are prevalent in the H9 schists, where intercalated boudinage lenses of gray limestone are present. Identification of this carbonate as limestone lenses differs from interpretations by previous workers (Liu, 1986), who considered them to be carbonatite dikes. Mineralized H9 biotite schist locally contains magnetite and REE minerals (Chao and others, 1992).
The Bayan Obo ores are complex, as is indicated by the number of minerals listed, which range from 120 to 174, according to various reports (Bai and Yuan, 1985; Zhang and Tao, 1986; Institute of Geochemistry, 1988; Chen and others, 1990). These minerals (table 1) consist of both primary and secondary metamorphic minerals of the host rocks and the epigenetic ore and gangue minerals introduced by hydrothermal solutions. Most of the Bayan Obo minerals, particularly in the banded ore, are very fine to extremely fine grained, late-stage minerals; the minerals in veins cutting the previously formed ores are coarser grained. Typically, Bayan Obo hand specimens show a variety of colors that are characteristic of some of the most abundant minerals. Among these are pale-yellow monazite, yellow to brownish-yellow bastnaesite, pale- to dark-blue alkali amphibole, yellow phlogopite, pale-green to green aegirine and aegirine augite, colorless to lavender-purple to dark-purple fluorite, bluish-gray to gray hematite and martite, and dark-gray to black magnetite.
REE, Fe, and Nb ore minerals have been identified in the Bayan Obo ores. The most abundant REE minerals are very fine grained monazite and bastnaesite and coarser grained huanghoite (table 1). Fine- to medium-grained bastnaesite also occurs in late-stage ore dominated by aegirine and aegirine augite, where it is associated with apatite and barite. Huanghoite grains generally are larger, up to 2-3 cm in diameter, and commonly are associated with medium- to coarse-grained aegirine-rich ores. Cordylite (Ba(Ce,Nd,La)2(CO3)3F2) is similar to huanghoite in composition (table 1) but differs in usually being very fine grained. Less common REE minerals are parisite and cebaite (table 1). The monazites and bastnaesites of Bayan Obo ores are typically very low in U (less than 5 ppm) and low in Th (less than 7,000 ppm). Columbite is the most abundant Nb mineral, and aeschynite, fergusonite, fersmite, and Nb-rutile are common Nb-bearing minerals. Pyrochlore and other Nb- and REE-bearing minerals such as chevkinite ((Ca,Ce,Th)4(Fe,Mg)2(Ti,Nb,Fe)3Si4O22) are rare. The most important Fe minerals in the ores are magnetite, primary fine granular hematite, and martite (pseudomorphs of hematite after euhedral magnetite). Goethite is present in altered zones associated with both primary magnetite- and hematite-rich ores. Ilmenite is present locally, particularly in the late-stage banded ores.
Ba-bearing and Sr-bearing carbonate minerals such as benstonite, norsethite, barytocalcite, strontianite, and Sr-calcite (table 1) occur as late-stage minerals replacing carbonate host rocks along grain boundaries. The Ba-bearing carbonate minerals appear to be unrelated to barite, which is one of the most common gangue minerals.
The most widespread gangue minerals are alkali amphiboles (predominantly magnesio-arfvedsonite), fluorite, barite, aegirine augite, phlogopite, apatite, albite, and microcline. Alkali amphiboles provide information about the episodes of hydrothermal activity and regional metamorphism. One of the most distinctive characteristics of alkali amphiboles in Bayan Obo is that they appear to be unrelated to REE mineralization, unlike the amphiboles in carbonatites. Several generations of magnesio-arfvedsonite, and its varied chemical contents of FeO, MgO, MnO, K2O, and Na2O, can be distinguished on the basis of textural evidence and 40Ar/39Ar ages. Magnesio-riebeckite, also present, is more common in veins than magnesio-arfved-sonite and occurs as overgrowths on prismatic magnesio-arfvedsonite cores, such as shown in sample 7B34-1.
Fluorite, on the basis of textural relations, usually replaces REE ore minerals such as monazite and bastnaesite and Fe minerals such as martite and granular hematite. Fluorite and barite commonly replace aegirine in the banded assemblages. Late-stage fluorite and barite also occur as veinlets cutting banded ores.
Aegirine augite replaces host rock dolostone and limestone marble, but more commonly, it occurs together with fluorite in banded ores. Although textural evidence indicates there may be more than one generation of aegirine and aegirine augite, the chemical composition of aegirine augite appears to be fairly uniform (see representative composition in table 5). Because aegirine augite and alkali amphibole usually do not occur in the same mineral assemblage, the time and space of formation of these two minerals were generally unrelated.
Table 5. Chemical composition (wt. percent) of huanghoite, apatite, and aegirine augite of Bayan Obo.
Phlogopite, commonly associated with the various generations of alkali amphiboles, replaces host dolostone or limestone marble. Phlogopite and alkali amphiboles partially replaced the host marble as well as the fine-grained monazite and bastnaesite that had previously partially replaced this marble, and were subsequently partially replaced by late-stage unmartitized magnetite.
Medium-grained detrital apatite occurs in coarse-grained H8 dolostone marble. Fine-grained to very fine grained granular apatite and porphyroblastic apatite of hydrothermal origin generally are associated with small amounts of monazite and granular hematite in banded low-grade REE ores. The detrital apatite can be distinguished from very fine grained to fine-grained hydrothermal apatite by distribution, origin, and REE content. EPMA and SXRF analyses by Chen and others (1993) indicate that the light rare earth element (LREE) concentrations in detrital apatite in coarse-grained dolostone marble sample 7B25-1 are 3-6 times lower than those in hydrothermal apatite in banded ore sample 7B31-9 (table 5).
Albite is widespread, especially as a replacement mineral in the H9 biotite schist, where albite usually preceded and was replaced by late-stage unmartitized magnetite. Albite also occurs in veinlets in some banded ores and in late-stage albite-hornblende veins cutting H9 pelitic host rocks. Late-stage K-feldspar, usually microcline, occurs as an isolated accessory gangue mineral in some of the banded ores and is a common metasomatic mineral replacing H9 biotite schist (Drew and others, 1990).
Bafertisite and cerianite are accessory minerals at Bayan Obo (table 1). Bafertisite typically occurs in aegirine magnetite ores as a prismatic, bright-orange, strongly pleochroic accessory mineral. Cerianite, an alteration product (Zhang and Tao, 1986), is bright red in hand specimen, is red and isotropic in DPTS, and is the only Bayan Obo mineral radioactive enough to require shielding and caution in handling of samples that contain it.
Bayan Obo ores contain only sparse sulfide minerals, principally pyrite, pyrrhotite, sphalerite, galena, and chalcopyrite. Pyrite occurs as scattered disseminated grains in the H8 carbonate host, commonly associated with disseminated barite and trace amounts of fluorite. Pyrite also occurs as fine stringers and veinlets crosscutting banded ores. The other sulfides are late-stage minerals that occur in some disseminated ores and in small amounts in banded ores.
There are three broad categories of ore types: disseminated, banded, and massive. The classification of Bayan Obo ores used in this report is based on the classification developed in the early 1960's, which was published by the Institute of Geochemistry (1988) and is, generally, the classification currently used by mine geologists. The classification is based on three principal parameters: (1) field relations and the type of host rock replaced by the ore and gangue minerals, such as H8 dolostone or limestone marble, quartzite, and H9 biotite schist and albitized biotite schist; (2) textures and structures, whether the ore minerals are disseminated, banded, or massive; and (3) mineral assemblages; presence or absence of REE minerals, in particular, monazite and bastnaesite; and types of Fe-oxide minerals and dominant gangue minerals. The following accessory minerals, present only in minor amounts, are not included in the classification scheme: barite and Ba-bearing REE minerals such as huanghoite, cordylite, and cebaite; scattered apatite; ilmenite; bafertisite; microcline; pyrite and other sulfides, such as chalcopyrite, sphalerite, and pyrrhotite; Nb-bearing minerals such as columbite, Nb-bearing ilmeno-rutile, fersmite, aeschynite, and fergusonite; and other rare minerals such as barytocalcite, benstonite, and strontianite. Where relicts of host rock are not observed, the ore classification is based only on textural-structural criteria and mineral assemblages.
The distribution of ore in the Main and the East Orebodies is shown in figure 8, the outlines of the orebodies being based on average Fe contents of at least 20 wt. percent. The disseminated dolomite-hosted REE ores, from less than 2 to more than 6 wt. percent total RE2O3, are distributed generally at or below the footwalls of the Main and East Orebodies and above the hanging wall of the East Orebody (fig. 8). Massive Fe ores, made up of various Fe-oxide minerals, occur in the central parts of both orebodies and are surrounded by banded ores, which are the dominant type of ore in the orebodies. This broad zoning was recognized by mine geologists as early as the 1960's.
Figure 8. Geologic sketch map of the Main and the East Orebodies of Bayan Obo (modified from Institute of Geochemistry, 1988).
The following classification scheme, which is used in this report, includes some ore types that are not mappable units and, therefore, are not delineated in figure 8.
a. Marble with disseminated monazite only (sample 7B36-6; sample locations are shown in fig. 5), with bastnaesite only (samples 34-24, not shown in fig. 5, and 8B7), or with monazite and small amounts of bastnaesite (sample 796-48,278). Locally, fluorite is an abundant gangue mineral.
b. Marble with disseminated monazite and magnetite or martitized magnetite (samples B494-89-98, not shown in fig. 5, and 7B36-1B), with or without fluorite.
c. Marble with disseminated granular hematite, with or without small amounts of magnetite (samples 7B36-12 and 9B4), and with or without gangue minerals such as fluorite and barite.
d. Marble with disseminated monazite or bastnaesite and magnetite, and significant amounts of extremely fine grained granular aegirine augite, with or without fluorite (sample 796-48, 459).
e. H8 marble with disseminated monazite, bastnaesite, magnetite, and the gangue minerals phlogopite and alkali amphibole (mainly magnesio-arfvedsonite) (samples 796-48,338 and 796-48,277).
f. H8 limestone marble with disseminated monazite closely associated with significant amounts of extremely fine grained granular apatite, with or without bastnaesite, magnetite, and granular hematite (sample 398-46,305).
a. Monazite (+-bastnaesite)-granular hematite-fluorite banded ore (type FH, figs. 8 and 9). This type of banded ore is widespread and consists mainly of varying proportions of alternating submillimeter- to millimeter-thick discontinuous laminae and bands consisting principally of extremely fine grained monazite, extremely fine grained granular hematite, and fluorite. There are many varieties of monazite-hematite-fluorite banded ores.
b. Aegirine augite-magnetite banded ores, with or without varying amounts of monazite or bastnaesite (type AeM, figs. 8 and 10). There are many varieties of this type of banded ore. Where aegirine augite is the dominant gangue mineral, apatite is common, bastnaesite is usually the dominant REE mineral, and martite or magnetite is the dominant Fe ore mineral.
c. Mixture of types a and b, consisting of monazite, granular hematite, and magnetite, and both aegirine and fluorite as associated gangue minerals (fig. 11). Banded ore that contains both fluorite and aegirine gangue minerals is the most abundant and widespread type of banded ore at the Main and East Orebodies of Bayan Obo. Because this ore type does not occur as a mappable unit, it is incorporated in banded ore types 2a (FH) and 2b (AeM) shown in figure 8.
d. Monazite-phlogopite-alkali amphibole-magnetite banded ore (type AmpM, fig. 8), with or without fluorite and aegirine augite. This type of banded ore is closely related to the disseminated ore that has a similar mineral assemblage (type 1e).
e. Granular apatite-granular hematite-monazite banded ore (type Ap, figs. 8 and 12) that usually contains fluorite. This ore type, characterized by the presence of granular apatite, is closely related to disseminated ore that has a similar mineral assemblage (type 1f).
Massive Fe ores consist principally of at least one of the Fe-oxide minerals granular hematite, martite, and magnetite (fig. 13). They represent more intense Fe mineralization than either disseminated or banded ores. Massive Fe ores occur in H8 carbonate and quartzite hosts and H9 schist; they also replace banded ores of known or unknown host rocks (samples 7B32-1 and 7B40-2).
The interbedded quartzite in the H8 carbonate unit makes up a very minor part of the H8 and is mineralized only in the Main and East Orebodies, particularly along the footwall of the East Orebody (fig. 8). In spite of the resistance of quartz to chemical alteration, the types of mineralization of quartzite-hosted ores appear similar to those hosted by the H8 carbonates.
The most common quartzite-hosted disseminated ore contains extremely fine grained aegirine augite, bastnaesite, and barite, with or without varying amounts of magnetite (sample 9B18-3c). Some of these ores also contain various amounts of fluorite.
a. Bastnaesite-granular hematite-fluorite banded ore with or without minor granular aegirine (samples 7B30-39, 7B30-40, 7B30-41A, and 7B30-41B). This type of banded ore occurs at the footwall in the northeastern part of the East Orebody (fig. 8).
b. Aegirine-magnetite banded ore, with or without bastnaesite and monazite (sample 7B30-43).
c. Aegirine-fluorite-magnetite ore, with a variety of Ba-bearing REE fluorcarbonate minerals, such as huanghoite and cebaite; barite is common (samples 7B30-47, 9B2-5-1, 9B13-2, 398-46,142).
d. Granular apatite-granular hematite banded ore, with varying amounts of monazite (samples 9B12a, 8B35-8).
Quartzite-hosted massive martite ores with granular hematite contain relict quartz (samples 7B39-4, 398-46,335). Quartzite-hosted massive granular hematite ore is also present, but apparently is not a major ore type.
The ores hosted in biotite schist occur only in the hanging wall of the Main Orebody (fig. 8). The most common type of ore in biotite schist is aegirine augite-biotite-magnetite disseminated ore (sample 9B5-2). Locally, small amounts of REE minerals such as cebaite occur in low-grade, disseminated magnetite ores dominated by aegirine augite.
Figure 9. Hand specimen (sample 7B30-1) of typical microfolded banded ore consisting of irregular to lenticular layers of monazite, granular hematite, and fluorite.
Figure 10. Large block of banded ore consisting of layers of aegirine augite and magnetite. Sample 8B13 excavated from bench level 1558 m, at the east end of the East Orebody.
Figure 11. Block (sample 9B3i) showing both fluorite and aegirine augite as major gangue minerals.
Figure 12. Block (sample 9B13) of banded ore consisting of granular apatite, granular hematite, and monazite.
Figure 13. Outcrop on bench 1582 m of the East Orebody, showing bodies of massive magnetite and hematite ores.
Selection of ore types for chemical analysis was based on mineral assemblages. Two different billets of each banded ore sample to be analyzed, each approximately 2-3 cm wide, 4-6 cm long, and 0.5 mm thick, were cut across the banded structure. A DPTS was made from one billet, and the other billet was submitted for chemical analysis (tables 6-12). Two DPTS's (parts A and B) were needed for large samples, which extended for a distance of more than 6 cm across the bands. Sample locations are shown in figure 5.
Table 6. Partial chemical composition (wt. percent) of disseminated monazite ore in dolostone marble and of monazite-hematite-apatite banded ores of Bayan Obo.
Table 7. Partial chemical composition (wt. percent) of selected monazite (bastnaesite)-fluorite (low-Fe, high-REE) banded ores of Bayan Obo.
Table 8. Partial chemical composition (wt. percent) of bastnaesite-apatite-aegirine augite-fluoritemartite banded ores with apatitebarite of Bayan Obo.
Table 9. Partial chemical composition (wt. percent) of high-Fe, REE, and fluorite banded ores of Bayan Obo.
Table 10. Partial chemical composition (wt. percent) of high-REE, low-Fe, and low-F banded ores of Bayan Obo.
Table 11. Partial chemical composition (wt. percent) of marble-hosted alkali amphibole-magnetite banded ores of Bayan Obo.
Table 12. Partial chemical composition (wt. percent) of quartzite-hosted ores of Bayan Obo.
Because of the high contents of combined F, Fe, and REE's in many of the ore types, only partial analyses of ore samples could be obtained by the ICP-AES method, which did not analyze for SiO2, Na2O, CO2, or in most cases, P2O 5; therefore, low analytical totals are reported in tables 6-12. Total Fe is reported as Fe2O3 (tables 6-12), because ferric Fe is the dominant form of Fe in most of the minerals present. Each sample of ore from a quartzite host has a low total of constituents because SiO2 content was not determined. Similarly, for a sample containing aegirine augite, the total is low because neither SiO2 nor Na2O was determined. Because of the heterogeneity and erratic variation of mineral assemblages in bands in banded ores, the chemical composition of an ore is not expected to correlate with the modal mineral composition of the corresponding DPTS. Nevertheless, the general mineral assemblage of the DPTS does reflect the chemical composition qualitatively.
Partial analyses of selected banded ores from Bayan Obo (tables 6-12) are divided on the basis of mineral assemblages from DPTS studies and concentrations of REE's and Fe and are discussed as follows:
1. Dolostone marble having relatively high REE content, and monazite-hematite-apatite banded ores (table 6).
Partial analyses of the chemical compositions of two fractions of a high-grade REE-disseminated ore sample, 7B30-33 (7B30-33A and 7B30-33B, table 6), each having significant amounts of relict host carbonate, show contents of RE2O3 of 19.56 and 13.23 wt. percent, respectively. Sample 7B30-33A contains more REE's (monazite) and Fe (granular hematite) and has less fluorite than 7B30-33B. Partial chemical analyses of monazite-hematite-apatite banded ores are also shown in table 6. The bands of granular apatite visible in the corresponding DPTS's are not reflected in the analyses because P2O5 concentrations were not determined. Sample 7B30-27 contains more Fe (total Fe2O3 is 18.6 wt. percent, occurring as granular hematite) and REE's (total RE2O3 is 8.28 wt. percent) than sample 7B37-3. The CaO present in both samples is due to apatite and fluorite. The host rock, probably dolostone marble, has been completely replaced by gangue and ore minerals.
2. Monazite (+-bastnaesite)-fluorite (low-Fe, high-REE) banded ores (table 7).
The high content of fluorite in the fluorite-type banded ores is indicated by the high F and CaO contents in the analyses (table 7). RE2O3 contents of the seven samples analyzed range from 12.29 to 25.06 wt. percent. Samples 7B31-1A and 7B31-1B from successive bands of the same hand specimen are similar in composition, but the total RE2O3 content of 7B31-1B (25.06 wt. percent) is appreciably higher than that of 7B31-1A (18.27 wt. percent). The presence of small amounts of MgO in these samples is probably due to the presence of phlogopite observed in the hand specimen, whereas Fe comes from fine granular aegirine augite and magnetite.
3. Bastnaesite-apatite-aegirine augite-fluorite+-martite banded ores (table 8).
These ores are characterized by the presence of finely granular aegirine augite, in dominant bands, and variable amounts of fluorite. The principal REE mineral is very fine grained to medium-grained bastnaesite. Chemically, aegirine augite and local, sparse martite are reflected by the total Fe present (Fe2O3 ranges from 6.86 to 20.0 wt. percent), whereas fluorite is reflected by the Ca and F contents (CaO ranges from 2.94 to 37.78 wt. percent and F ranges from 1.02 to 16.0 wt. percent). The total RE 2O3 content ranges from 3.05 to 17.45 wt. percent (table 8).
4. High-Fe, REE, and fluorite banded ores (table 9).
Samples 7B30-28A and 7B30-28B are from the same hand specimen, which consists of bands containing mixtures of magnetite, monazite, bastnaesite, apatite, and fluorite. Magnetite accounts for the total Fe present (24.31-37.17 wt. percent Fe2O3) in this sample, and monazite and bastnaesite account for the total RE2O3 present (10.89-22.36 wt. percent). The differences in Fe and REE content between 7B30-28A and 7B30-28B reflect differences in the abundance of Fe versus REE bands in the samples analyzed. Sample 7B30-37 is a dolostone marble replaced by granular hematite and trace amounts of monazite. Sample 7B31-14 is a banded Fe-REE ore sample containing bands of granular hematite, REE minerals (monazite and bastnaesite), and fluorite. It has 35.7 wt. percent Fe2O3 and 17.80 wt. percent total RE2O3.
5. High-REE and low-Fe and low-F banded ores (table 10).
These banded ores consist of prominent bands containing abundant monazite and bastnaesite. Fluorite and other gangue minerals, such as aegirine in 7B31-10 and apatite in 7B31-23 and 7B37-1, may be present. The RE2O3 contents of the four samples listed in table 10 range from 36.42 to greater than 48.10 wt. percent. Although samples having such high REE content are not rare at Bayan Obo, most of the REE ores contain less than 20.0 wt. percent total RE2O3 (tables 6-9).
6. Marble-hosted alkali amphibole-magnetite banded ores (table 11).
This ore type occurs near or at the hanging wall of the East Orebody (figs. 5 and 8). The ores have rather low (3.35-4.61 wt. percent) total RE2O3 contents and are characterized by a rather high MnO content (3.49-5.42 wt. percent) and high Fe content due to the presence of late-stage magnetite. Fluorite is generally present, as indicated by the 7.18-9.58 wt. percent F in the samples.
7. Quartzite-hosted ores (table 12).
The samples in table 12 are from a mineralized bed of quartzite along the footwall of the northeastern part of the East Orebody (figs. 5 and 8). The total values in table 12 are low because SiO2 contents of the host quartzite and silicate gangue minerals were not determined by the ICP-AES method of analysis. Aegirine augite and fluorite are generally present in varying amounts; alkali amphibole is typically absent from ores where aegirine augite is an essential component. The dominant REE mineral is bastnaesite, but lesser amounts of cebaite also are present.
8. Massive Fe ores consisting of granular hematite and martite or of magnetite or of mixtures of Fe-oxide minerals.
Although these three ores were not analyzed, according to mine geologists, massive Fe ores may contain as much as 55 wt. percent Fe. Most massive Fe ores contain less than 3 wt. percent RE2O3.
Multiple lines of evidence are needed in order to determine the origin and the mineral paragenetic sequence of the Bayan Obo ore deposit (Chao and others, 1993). Replacement textures observed in the field and petrographically from DPTS's are the most important criteria, and are aided by corroborative criteria such as chemical compositions and radiometric isotopic ages of various generations of ore and gangue minerals. An absolute mineral paragenetic sequence can be partially established on the basis of absolute ages of episodes of REE mineralization and the unique chemical compositions and isotopic ratios of the different generations of REE minerals. Without the age and compositional data of key ore and gangue minerals, only the relative mineral paragenetic sequence of complex mineral assemblages can be inferred. For this reason, episodes of REE mineralization as a whole are well established, whereas episodes of Fe and Nb mineralization are not. On the basis of widespread metasomatic replacement textures, and chemical compositions and radiometric isotopic ages of ore and gangue minerals, the generalized mineral paragenetic sequence of Bayan Obo was determined (table 13).
Table 13. Generalized mineral paragenetic sequence of Bayan Obo.
Only a few examples of textural evidence of metasomatic replacement at Bayan Obo are illustrated here. These are typical or representative features of replacement of host rock (marble, quartzite, and biotite schist) by either an ore or a gangue mineral, as well as later replacement of an earlier replacement mineral.
There is widespread evidence of host dolostone marble replacement by various ore minerals, such as monazite, bastnaesite, martitized magnetite, fine granular hematite, and fresh magnetite. In the field (fig. 14), a block of light-colored H8 dolostone marble was partly replaced by disseminated gray magnetite. Typically, the Proterozoic host H8 dolostone marble was granulated and recrystallized by regional metamorphism before being replaced interstitially by extremely fine grained monazite of Caledonian age (fig. 15).
Figure 14. Block of H8 dolostone marble partly replaced by disseminated magnetite.
Figure 15. Photomicrograph of a doubly polished thin section (sample 7B36-'90) of the host dolostone marble showing granulation texture and interstitial replacement of granulated grains of dolomite by very fine grained monazite.
Also widespread was the metasomatic replacement of host rocks by gangue minerals such as fluorite, aegirine augite, phlogopite, alkali amphibole, fine granular apatite, albite, and microcline. Hydrothermal replacement of marble by fluorite is observed from initial interstitial replacement to the development of bands of fluorite in the host rocks (fig. 16) to terminal stage, where the host dolomite occurs as scattered islands completely surrounded by fluorite. Extremely fine grained aegirine augite similarly replaced host dolomite. Terminal-stage replacement of marble, where relict dolomite grains occur completely surrounded by extremely fine grained aegirine augite, is shown in figure 17.
Figure 16. Photomicrograph of a doubly polished thin section (sample 7B30-2) of banded fluorite dolostone marble showing fluorite replacement of dolomite at triple-junction grain boundary.
Figure 17. Photomicrograph of a doubly polished thin section (sample 796-48,449) of marble showing relict dolomite grains surrounded by fine granular aegirine augite that partially replaced dolomite.
After more than 2 years of testing, the 232Th/208Pb internal isochron method was adopted for dating REE minerals (Wang and others, 1994). This method is preferred over other methods because precise ages are obtained on the basis of actual measurements of Th and Pb, rather than by assuming initial Pb concentrations. A total of 14 monazite samples and 3 bastnaesite samples were separated from Bayan Obo disseminated and banded ores by the Tianjin Geological Research Academy laboratory in Tianjin and the USGS laboratory in Reston, Va. The purity of single samples of monazite or bastnaesite was then checked by using SEM-EDX in the Reston laboratory before submitting the samples to the USGS laboratory in Denver, Colo., for isotopic determination. Each of the single samples of the REE minerals was further separated into several fractions for isotopic analysis by using a Franz magnetic separator equipped with an alcohol chute (Wang and others, 1994). The results for these 17 samples were described by Wang and others (1994). Two pieces of evidence suggest that the 232Th/208 Pb data are reliable: (1) all of the monazite and bastnaesite samples submitted yielded good internal isochrons, so that none were rejected; and (2) the internal isochron ages of two splits of monazite sample 796-48,278, submitted 3 months apart to the USGS Denver laboratory, are 422+-16 and 415+-8 Ma, respectively, their combined internal isochron age being 419+-3.7 Ma (fig. 18). Each split of sample 796-48,278 is represented by five data points. The isotope dilution analyses of the two splits are, respectively, 1,767 and 1,770 ppb total U, 2,830 and 2,449 ppm total Th, and 54.26 and 48.96 ppm total Pb. A few internal isochrons are presented here to help illustrate that different generations of monazite and bastnaesite differ not only in their chemical composition and age, but also in initial 208Pb/204Pb ratios, which probably represent different hydrothermal solutions.
Figure 18. Internal isochron showing mineral age of two aliquots of monazite from sample 796-48,278. The lower five data points are based on Th/Pb isotopic ratios of one aliquot, and the upper points are for the other aliquot.
Chemical compositions of monazites and bastnaesites were obtained by EPMA, using various calibrated standards. EPMA data on LREE's of monazite and bastnaesite (table 14) are in general agreement with the published average chemical composition of 17 monazite and 22 bastnaesite samples from Bayan Obo, determined by wet chemical methods (Zhang and Tao, 1986, p. 99). EPMA data on REE content of apatites (table 5) are also in good agreement with data obtained by the SXRF method (Chen and others, 1993). Although the 17 REE mineral samples that were dated were also analyzed using INAA for all REE's, including the heavy REE's (HREE's), the INAA data showed significantly lower amounts of LREE's when compared with the EPMA data. Research is in progress to account for this discrepancy. Because the amounts of HREE's are too low to be analyzed by the EPMA method, complete REE distribution curves are not available. EPMA of six of the monazites shows amounts of Y2O3 ranging from 0.12 to 0.39 wt. percent. Further studies may indicate whether recovery of Y from this ore deposit is economical.
Table 14. Partial chemical composition and internal isochron mineral ages of monazites and bastnaesites of Bayan Obo.
Concentrations of LREE's (La, Ce, Nd, and Pr), U, Th, and Pb and initial 208Pb/204Pb ratios constitute the eight chemical parameters used to distinguish monazites and bastnaesites in this study. The chemical compositions and internal isochron ages of monazites and bastnaesites are given in table 14, arranged according to internal isochron ages. The earliest bastnaesite and monazite mineralization in disseminated marble ores occurred at 555+-11 Ma (sample 8B7) and 553+-16 Ma (sample 7B36-6), respectively. The bastnaesite sample 8B7 (table 14) is characterized by a relatively low La2O3 content (22.92 wt. percent) and a high Nd2O3 content (10.53 wt. percent) compared with other dated bastnaesite samples analyzed. The bastnaesite of this generation is also characterized by low U, Th, and Pb concentrations, as well as a low initial 208Pb/20 4 Pb ratio. The monazite in sample 7B36-6 is characterized by uniquely low La2O3 content (13.26 wt. percent), very high Nd2O3 (11.27 wt. percent), high Pr2O3 (4.06 wt. percent), extremely high Ce2O3 (41.55 wt. percent, the highest reported), and a moderately high 208Pb/204Pb initial ratio (39.76+-2.06). Disseminated monazite from dolostone marble sample 34-24 (table 14, fig. 19), with an internal isochron age of 532+-3.1 Ma, is chemically characterized by a moderately low La2O3 content (17.63 wt. percent), a moderately high Nd2O3 content (10.80 wt. percent), and a low 208Pb/204Pb initial ratio (34.89+-0.15).
Figure 19. Internal isochrons of monazite from two samples (490,330 and 34-24) of Bayan Obo ores showing ages and 208Pb/204Pb initial ratios.
Bastnaesite from banded ore sample 796-48,280 (table 14), with an internal isochron age of 496+-13 Ma, is characterized by a high La2O3 content (28.64 wt. percent) and a low Nd2O3 content (7.83 wt. percent). Monazite from banded monazite-granular hematite-fluorite sample 8B35-2, with an internal isochron age of 474+-4.5 Ma, is characterized by low Nd2 O3 content (8.37 wt. percent), low Ce2O3 (33.84 wt. percent), relatively high total U (3,384 ppb), and a moderately high 208Pb/204Pb initial ratio (40.17+-0.70). Disseminated bastnaesite from dolostone marble sample 85-10,21-4 (table 14), with an internal isochron mineral age of 475+-8 Ma, is chemically characterized by a moderate La2O3 content (24.89 wt. percent), a high Nd2O3 content (10.90 wt. percent), and an unusually high 208Pb/204Pb initial ratio (59.83+-5.40). Although the monazite from 8B35-2 and bastnaesite from 85-10,21-4 are nearly identical in their internal isochron age, they probably were not deposited from the same hydrothermal solution because their initial 208Pb/204Pb ratios are very different.
Monazite samples from banded ores of diverse chemical compositions have internal isochron ages ranging from 474+-4.5 Ma (sample 8B35-2, table 14) to 404+-14 Ma (sample 9B9-3, table 14); the peak of REE mineralization of the Bayan Obo ores occurred between 430 and 420 Ma. Monazite sample 9B9-3 (table 14), with an internal isochron age of 404+-14 Ma (the youngest monazite from banded ores), has the highest total U content of 7,630 ppb. The youngest monazite from marble-hosted disseminated ore sample 12-6A, has an internal isochron age of 398+-9.7 Ma and the highest total Th content (6,071 ppm) of all REE samples analyzed. Only two samples, 7B31-2 and 7B31-9 (table 14), are nearly identical with respect to all eight chemical parameters. Samples 796-48,270 and 796-48,278 (table 14) are similar in LREE composition and age, but the large uncertainties regarding the age and 208Pb/204Pb initial ratio for sample 796-48,270 make comparison between these two samples difficult.
The internal isochrons, in ascending order of 208Pb/204Pb initial ratios, for selected samples are shown in figures 19-22. Monazite from sample 490,330 (fig. 19), with an internal isochron mineral age of 431+-2.7 Ma, has the lowest 208Pb/204Pb initial ratio (32.47+-0.17) and is the most primitive, least radiogenic sample. Monazite from sample 34-24, with an internal isochron mineral age of 532+-3.1 Ma, is the next least radiogenic and has a 208Pb/204Pb initial ratio of 34.89+-0.15. Monazite from sample 7B36-6 (fig. 20) has an internal mineral isochron age of 553+-16 Ma and a 208Pb/204Pb initial ratio of 39.76+-2.06 that is indistinguishable from the 208Pb/204Pb initial ratio of monazite sample 8B35-2 (fig. 20), 40.17+-0.70. The internal isochron age of monazite sample 8B35-2 is 474+-4.5 Ma, clearly younger than 553+-16 Ma, and may indicate that the same or similar source reservoir for REE's was tapped and mobilized at different times. Monazites from samples 7B31-2 and 7B31-9 are similar in REE chemical composition (table 14) and indistinguishable in internal isochron age and 208Pb/204Pb initial ratios (fig. 21). Therefore, it is probable that the monazites from 7B31-2 and 7B31-9 originated concurrently from hydrothermal solutions derived from the same source reservoir. These two samples were collected more than 50 m apart, about 130 m above the footwall of the East Orebody (fig. 5).
Figure 20. Internal isochrons of monazite from two samples (7B36-6 and 8B35-2) of Bayan Obo ores that show nearly identical 208Pb/204Pb initial ratios indicate different internal isochron ages, suggesting a common source reservoir that was tapped at different times.
Figure 21. Internal isochrons of monazite from two samples (7B31-2 and 7B31-9) of Bayan Obo ores that have essentially the same internal isochron mineral ages and 208Pb/204Pb initial ratios.
Bastnaesite from sample 85-10,21-4 (table 14 and fig. 22) has an internal isochron mineral age of 475+-8 Ma and a very high 208Pb/204Pb initial ratio of 59.83+-5.40. Monazite from sample 7B30-20 (table 14 and fig. 22), with an age of 421+-3.3 Ma, has one of the lowest La2O3 contents and the highest Nd2O3 content, as well as one of the highest, most radiogenic 208Pb/204Pb initial ratios (66.65+-5.32). These two REE samples came from different hydrothermal solutions, not only different from each other but also from the solutions giving rise to the rest of the analyzed REE samples.
Figure 22. Internal isochrons of bastnaesite from sample 85-10,21-4 and monazite from sample 7B30-20 having different internal isochron mineral ages but high radiogenic 208Pb/204Pb initial ratios.
Spatial relations of REE mineralized samples are generally lacking. Four of the dated samples are however from the same drill core. They are 796-48,270, 796-48,277, 796-48,278, and 796-48,280 (table 14). Except 796-48,280 (depth 280 m), where bastnaesite is the REE mineral dated with an internal isochron age of 496+-13 Ma, the other three are monazite samples, from depths of 270, 277, and 278 m, respectively. Monazite from 796-48,270, 415+-17 Ma, is from a banded ore hosted by dolomite marble and associated with granular aegirine, magnetite, phlogopite, and fluorite; monazite from sample 796-48,277, 432+-8.5 Ma, is from a disseminated ore hosted by limestone marble and associated with abundant alkali amphibole, magnetite, and small amounts of aegirine, fluorite, and pyrite; and monazite from 796-48,278, 419+-3.7 Ma, is also from a disseminated ore hosted by limestone marble and associated with alkali amphibole, magnetite, fluorite, and pyrite. None of these three monazites can be distinguished readily by age, chemical composition, or initial 208Pb/204Pb ratio (owing to large uncertainties). These three samples may be considered to be of similar age and deposited by the same hydrothermal source.
The internal isochron mineral ages of episodes of monazite and bastnaesite mineralization are shown in table 14 and figures 18-22, 32, and 33. The episodes of REE mineralization recorded so far ranged from 555 to 398 Ma, covering a geologic time span of more than 150 million years. Chao and others (1992, 1993), on the basis of preliminary partial internal isochron and chemical data, concluded that the role of REE remobilization from carbonate host rocks was improbable, and that the effects of low-grade Caledonian and Hercynian regional metamorphism and Hercynian granitic intrusion on the ages of REE mineralization were minimal. The extensive and refined detailed data presented by Wang and others (1994) and in this paper, particularly the more complete chemical, isotopic, and initial 208Pb/204Pb ratios, fully substantiate the earlier conclusions. Most monazites with different internal isochron ages, chemical compositions, and initial 208Pb/204Pb ratios were probably brought in at different times and deposited by different hydrothermal solutions.
In general, there are three major Fe-oxide minerals associated with Fe mineralization, each perhaps occurring in more than one generation. These are (1) euhedral martite or martitized magnetite grains scattered in disseminated ores, in scattered aggregates in bands of banded ores, and in massive ores; (2) fine-grained to very fine grained primary granular hematite that forms disseminated patches and scattered lenses in dolostone marble disseminated ores and in bands in banded ores, and also occurs in massive ores; and (3) nonmartitized magnetite in disseminated, banded, and massive ores. Massive Fe ores commonly consist of a combination of the above three Fe-oxide minerals. Additional, but minor, occurrences of Fe minerals are very fine grained disseminated magnetite with rims of hematite, rare wormy textures in hematitic ores, and goethite in weathered Fe-oxide ores. The rimmed and wormy textures may be of metamorphic origin.
Data on ages of Fe mineralization are fragmentary because the Fe-oxide minerals cannot be isotopically dated and we were not successful in identifying various generations of fresh magnetite in terms of trace-element contents. Hence, textural relations among Fe-oxide minerals and dated REE minerals and alkali amphiboles were used to help determine the relative paragenetic sequence for Fe mineralization. These data yielded age information regarding only the latest and not the earliest age of Fe mineralization, but indicated that euhedral martite and martitized magnetite formed before granular hematite in disseminated and banded ores, and fresh magnetite formed last in disseminated, banded, and massive ores.
Euhedral martite and martitized magnetite are apparently the oldest Fe ore minerals in ores hosted by dolostone marble. Martite and martitized magnetite crystals are euhedral crystals of magnetite completely or partially replaced pseudomorphously along crystallographic directions of the original magnetite host. Thus, they record a two-phase process: the deposition of euhedral magnetite and later pseudomorphous replacement of magnetite by hematite. Where martite or martitized magnetite occurs with granular hematite, such as in some of the banded REE-Fe ores, texturally, martite and martitized magnetite preceded the granular hematite. This relation is illustrated where preexisting martitized magnetic crystals blocked the formation of continuous bands of granular hematite (fig. 23) and where euhedral martite crystals are completely embedded in fine, granular hematite (fig. 24). We interpret these relations to indicate that euhedral magnetite crystals formed first and were pseudomorphously replaced by hematite along crystallographic directions. The martite locally blocked and was incorporated into the younger bands of granular hematite.
Figure 23. Photomicrograph of a doubly polished thin section of sample 8B2-2 showing euhedral martitized magnetite crystals blocking the continuous formation of later fine-grained hematite.
Figure 24. Photomicrograph of a doubly polished thin section of sample 7B37-1 showing euhedral martite crystals embedded in fine granular hematite.
Granular hematite represents the next type of Fe mineralization. Evidence for the earliest episodes of granular hematite mineralization is found in sample 8B35-2, where fine-grained hematite with associated Nb-rutile replaced monazite with an age of 474+-4.5 Ma (table 14), and therefore crystallized after that date. Unfortunately, a minimum age for the hematite cannot be determined. Similarly, granular hematite progressively replaced dated monazite in sample 7B30-20 (figs. 25 and 26) and is, therefore, younger than 421+-3.3 Ma, the radiometrically determined age of the monazite. The monazite of sample 7B30-20 has about equal concentrations of La2O3 (13.60 wt. percent) and Nd2O3 (13.61 wt. percent) (table 14), which are low for La2O3 and uniquely high for Nd2O3 at Bayan Obo. Because of the unique chemical composition of this dated monazite, it can be used as a guidepost for estimating relative ages of minerals formed, based on textural evidence, before and after this monazite crystallized. Hematite bands in ores were cut by alkali amphibole veins (sample 7B30-30) with an 40Ar/39Ar plateau age of 395.8+-6.2 Ma; hence, the age of mineralization of some of the primary granular hematite probably lies between 421 and 396 Ma.
Figure 25. Photomicrograph of a doubly polished thin section of sample 7B30-20 showing bands of granular monazite replaced interstitially by fine granular hematite.
Figure 26. Photomicrograph of a doubly polished thin section of sample 7B30-20 showing strings of rounded monazite surrounded and replaced interstitially by fine granular hematite.
The occurrence of fresh magnetite in folded banded ore and in undeformed massive Fe ores indicates that at least two generations of fresh magnetite ores are present at Bayan Obo and apparently postdate the granular hematite. The textural relation between early granular hematite and late magnetite can be seen in sample 7B31-6, where a veinlet of fresh magnetite crosscuts bands of monazite and fine, granular hematite in banded REE-Fe ore (fig. 27); in sample 9B15-1L, where banded granular hematite-monazite-fluorite ore was replaced and cut by fresh massive magnetite (fig. 28); and in sample 796-48,517, where relict bands of granular hematite and monazite were disrupted and surrounded by fresh magnetite (figs. 29A and B). These relations also indicate that massive magnetite ores formed after banded ores and are supported by the apparent textural zoning in the Main and East Orebodies, where massive Fe ores occur in the center of the orebodies and are surrounded by banded ores.
Figure 27. Photomicrograph of a doubly polished thin section of sample 7B31-6 showing a magnetite veinlet cutting a banded monazite-bearing granular hematite ore.
Figure 28. Photomicrograph of a doubly polished thin section of sample 9B15-1L showing a banded ore consisting of monazite-granular hematite-fluorite cut by late-stage, coarse-grained fresh magnetite aggregates.
Figure 29. Photomicrographs of a doubly polished thin section of sample 796-48,517 showing relict banded ore consisting of folded monazite and granular hematite surrounded and replaced by late-stage, fresh magnetite.
The 40Ar/39Ar ages of coexisting alkali amphiboles can further refine the ages of Fe mineralization and also add information about the long period of time the mine area was subjected to hydrothermal activity. For example, in sample 796-48,277, a drill hole sample of low-grade, disseminated monazite-magnetite ore that contains abundant alkali amphibole (figs. 30A and B), host limestone marble was replaced by aggregates of fine-grained monazite, which were in turn enclosed and replaced by alkali amphibole. Fresh magnetite replaced both monazite and alkali amphibole and is, therefore, younger than these phases. In this sample, the monazite was determined to be 432+-8.5 Ma (table 14 and fig. 31A) by the 232Th/208Pb internal isochron method, and the alkali amphibole (magnesio-arfvedsonite) has a plateau age of 343.1+-3.0 Ma (table 15 and fig. 31B), which is considered to be a minimum age, determined by the 40Ar/39Ar incremental heating method. The age and relatively flat spectrum for this metasomatic-metamorphic amphibole suggest that it has not been significantly reset by the younger Hercynian granitic intrusion or regional metamorphism. Because the magnetite replaces the alkali amphibole, the age of this magnetite is younger than the age of the alkali amphibole, about 343 Ma. Although the age difference between monazite and amphibole, about 90 m.y., is large, the ages are well documented and are consistent with the textural relations. In samples 8B7 and 34-24, alkali amphibole also replaced REE minerals, and the ages of the amphibole and REE phases have been determined. Bastnaesite in sample 8B7 is 555+-11 Ma (fig. 32A), and alkali amphibole in the same sample is 395.5+-4.4 Ma (table 15 and fig. 32B). In sample 34-24, the monazite is 532+-3.1 Ma (table 14), and the coexisting alkali amphibole, dated by the K-Ar method by the Tianjin laboratory, is 441+-11 Ma. In samples 796-48,277 and 8B7, there is a possibility that the alkali amphibole may have been reset by the replacement and mineralization of late-stage fresh magnetite. If so, then the determined minimum alkali amphibole ages date the age of magnetite mineralization. In sample 34-24, magnetite is not present; hence, resetting of the alkali amphibole age by late-stage magnetite-precipitating hydrothermal solutions is unlikely.
Table 15. Chemical composition (wt. percent) and minimum mineral ages of various generations of selected alkali amphiboles of Bayan Obo.
Figure 30. Photomicrographs of a doubly polished thin section of sample 796-48,277, a low-grade ore consisting of alkali amphibole (mga), monazite (m), and magnetite (mt), as well as relict grains of the limestone marble host rock. The alkali amphiboles contain inclusions of monazite. Monazite and alkali amphiboles are replaced and cut by fresh magnetite. A, Transmitted light; B, Reflected light.
Figure 31. Radiometric ages of minerals in sample 796-48,277: A, Internal isochron showing mineral age of the monazite (4328.5 Ma), which replaced the host limestone marble and occurs as abundant inclusions in alkali amphibole; B, 40Ar/39Ar plateau showing age (343.13.0 Ma) of the coexisting alkali amphibole that replaced the monazite.
Figure 32. Radiometric ages of minerals in sample 8B7: A, Internal isochron showing mineral age of the bastnaesite (555+-11 Ma), which replaced the host dolostone marble; B, 40Ar/39Ar plateau showing age (395.5+-4.4 Ma) of the coexisting alkali amphibole.
Because of the very fine grained nature of most of the Nb minerals and our limited resources, only fragmentary data on the occurrence of Nb-bearing minerals are presented here. The most important Nb mineralization in the mine area is associated with the West Orebodies, which are west of the Main Orebody. Most of the Nb minerals occur as disseminated ores. Nb-rutile is closely associated with granular hematite in banded ores near the footwall of the Main Orebody, and columbite (table 1), the main Nb ore mineral, is closely associated with fresh magnetite in disseminated magnetite ores hosted by marble and dominated by alkali amphibole. Aeschynite (table 1) is the only coarse-grained Nb mineral that occurs in veins and is associated with huanghoite and alkali amphiboles. Pyrochlore (table 1), which is the dominant and typical Nb mineral associated with igneous carbonatites, is not a common Nb mineral in Bayan Obo, except in marble in contact with the Hercynian granodioritic rocks in the eastern part of the mine area. Preliminary fragmentary data indicate that the 438.2+-25.1 Ma age of vein aeschynite (Chao and others, 1993), a niobate mineral, may represent the earliest known age of Nb mineralization. The latest age of Nb mineralization is based on the occurrence of pyrochlore in skarn marble that is in contact with Hercynian biotite granodiorite, which occurs about 2 km east of the East Orebody.
The regional metamorphic and hydrothermal histories and the evolution of the Bayan Obo ore deposit are very complex. A general representation of this development is presented schematically in figure 33. Data on episodes of REE and Fe mineralization have already been described. Alkali amphiboles were used to decipher the complex metamorphic and metasomatic episodes of Bayan Obo. Three types of alkali amphiboles, namely, metamorphic, metasomatic, and metasomatic-metamorphic, were dated. Alkali amphiboles of these three origins were differentiated on the basis of texture and abundance. Amphiboles of regional metamorphic origin occur as prismatic and fibrous or asbestiform crystals disseminated within or linearly aligned along foliation planes in the H8 marble host rocks, and usually show some sign of deformation, such as strong wavy extinction or bent asbestiform. Metasomatic alkali amphiboles generally occur in ore hosted by H8 and locally may replace more than 10-50 volume percent of the host rock. The alkalis, Fe, and Si necessary to form the amphibole were derived from hydrothermal solutions. Metasomatic-metamorphic alkali amphiboles are represented by large quantities of linearly aligned alkali amphiboles within and replacing the host marbles. Many metasomatic-metamorphic alkali amphiboles have magnesio-arfvedsonite prismatic cores thinly rimmed with magnesio-riebeckite (for example, samples 7B34-1P and 7B34-1F, table 15).
Figure 33. K-Ar and 40Ar/39Ar minimum mineral ages of alkali amphiboles of both metasomatic and metamorphic origin from samples of the Bayan Obo REE-Fe-Nb ore deposit, Inner Mongolia, China.
Samples representing different generations of these three types of alkali amphiboles vary in the FeO/MgO ratios and in amounts of MnO and alkalis. All Bayan Obo alkali amphiboles are characteristically low in Al2O3. Generally, the chemical compositions of metamorphic alkali amphiboles are richer in MgO than FeO, whereas the hydrothermal metasomatic alkali amphiboles are richer in FeO than MgO.
Previous and contemporary studies of Inner Mongolia indicate that several periods of regional metamorphism could have occurred in the Bayan Obo region. For central Inner Mongolia, Tang and Yan (1993) recognized four different periods of regional metamorphism--Middle Proterozoic, early Paleozoic (two stages), and late Paleozoic. The two stages of early Paleozoic metamorphism were (1) subduction zone metamorphism that resulted in paired metamorphic belts in the Ondor Sum (Wenduermiao) ophiolite (fig. 4) and the Bainaimiao island arc complex, and (2) orogenic metamorphism that occurred during the collision of an island arc with the continent (North China craton). According to Tang and Yan, the Middle Proterozoic and late Paleozoic metamorphic events were associated with rifting and were characterized by low-pressure metamorphic facies. Zhang and Tao (1986) recognized two stages of Hercynian granite rocks; biotite ages show the early stage of 311-306 Ma and the later stage of 258-234 Ma.
We recognize three stages of regional metamorphism (fig. 33) in the Bayan Obo mine region (Conrad and others, 1991; Chao and others, 1995): Late Proterozoic (about 900 Ma), Silurian to Mississippian (Caledonian, 425-343 Ma), and Pennsylvanian to Triassic (Hercynian, 310-220 Ma). On the basis of the presence of alkali amphiboles of metamorphic origin, the degree of metamorphism during the Late Proterozoic and Caledonian events was probably equivalent to low-grade amphibolite facies. For Hercynian rocks, the presence of metamorphic biotite indicates a greenschist facies of regional metamorphism.
Late Proterozoic regional metamorphism converted H8 carbonate to marble and sandstone to quartzite. The marble and quartzite show poorly to well-developed triple-junction grain boundary textures, the result of mylonitization and recrystallization. Coarse-grained ferroan dolomite in H8 dolostone marble was strongly twinned and stretched, and prismatic alkali amphiboles crystallized parallel to the direction of carbonate stretching within and along foliation planes in the marble. The minimum age of this event of regional metamorphism, based on single-grain, laser total-fusion 40Ar/39Ar analyses of lineated magnesio-arfvedsonite prismatic crystals in coarse-grained H8 dolostone marble (samples 40-14 and 8B24-1, table 15) is about 890 Ma. The chemical compositions of these magnesio-arfvedsonites from coarse-grained H8 dolostone marble are characterized by MgO>FeO, about 2-3 wt. percent K2O, and about 2.2-2.5 wt. percent F.
The Caledonian regional metamorphic event began at about 425 Ma and continued until at least 395 Ma, based on 40Ar/39Ar ages of amphiboles. The 425 Ma age is based on two types of metasomatic-metamorphic alkali amphiboles that occur in fine-grained to very fine grained, distinctly foliated, dolostone marble (samples 7B34-1F and 7B34-1P, table 15). One type of amphibole occurs as fine- to medium-grained prismatic magnesio-arfvedsonite crystals (7B34-1P) and the other as extremely fine grained asbestiform magnesio-riebeckite crystals (7B34-1F). The prismatic crystals are zoned or rimmed with magnesio-riebeckite that has a composition identical to the coexisting asbestiform magnesio-riebeckite. The 40Ar/39Ar plateau age of the prismatic crystals analyzed with the rim intact is 424.8+-2.4 Ma, and the 40Ar/39Ar plateau age of the asbestiform magnesio-riebeckite is 428.0+-7.3 Ma, which is indistinguishable from that of the prismatic amphibole. Because of the large amounts of alkali amphiboles present, particularly the asbestiform variety, we interpret this mineral occurrence to be of metasomatic-metamorphic origin. The chemical composition of the prismatic amphibole (sample 7B34-1P) is much more sodic than the asbestiform variety, which is distinctly more calcic; both varieties of amphibole are higher in MgO than FeO (table 15). Caledonian metamorphism continued to at least about 395 Ma, based on the 40Ar/39Ar plateau age of coarse, bent, asbestiform blue-green amphibole in a 2- to 3-cm-wide vein that crosscuts banded ore (sample 7B30-30, table 15). This amphibole is oriented normal to the wall of the vein, has a 40Ar/39Ar plateau age of 395.8+-6.2 Ma, and is characterized by a MgO content slightly higher than that of FeO. On the basis of the age of this alkali amphibole, the Bayan Obo banded ores are interpreted to have formed before 400 Ma.
In addition to the widespread occurrence of Hercynian biotite granites and granodiorites in the Bayan Obo region, evidence for the Hercynian regional metamorphism is based on 40Ar/39Ar single-grain total-fusion analyses of biotite from H9 biotite schist, which was reset and yielded ages ranging from 309.8+-5.8 to 297.1+-5.2 Ma. The 40Ar/39Ar plateau age of the Hercynian granodiorite, based on a single biotite sample, is 259.0+-3.1 Ma. A sample of K-feldspar from a pegmatite dike crosscutting H9 biotite schist gave a K-Ar age of 225+-7 Ma.
The occurrence of post-Caledonian or late Caledonian or early Hercynian low-grade regional metamorphism was also needed to account for the widespread presence of microchevron folds in banded ores and the recrystallized triple-junction grain boundary textures observed in some of the very fine grained REE and gangue minerals (apatite, barite, and fluorite) within bands of the banded ores. This period of low-grade regional metamorphism probably occurred before the main stage of fresh magnetite mineralization because fresh magnetite replaces folded banded ores, as shown in figure 29. Hercynian regional metamorphism also accounts for the isoclinal and chevron folds that are widespread in the H9 biotite schists with limestone boudinage. Clearly, the episodes of magnetite and Nb mineralization and the range of Hercynian regional metamorphism are complex and require more detailed study.
Magnesio-riebeckite from a vein crosscutting H6 quartzite (sample Drew 1, table 15) gave a disturbed 40Ar/39Ar spectrum inferred to reflect an age of 1.26 Ga (Conrad and McKee, 1992) and is the earliest evidence of hydrothermal activity in the Bayan Obo region. This magnesio-riebeckite is high in FeO (22.74 wt. percent) and low in MgO (10.28 wt. percent) and K2O (1.03 wt. percent). The next period of hydrothermal activity in the mine area that we were able to document is indicated by magnesio-arfvedsonite from a vein (sample 7B13C, table 15) that also crosscuts the H6 quartzite in the West Orebodies region, about 5.2 km west-southwest of the west end of the Main Orebody. Prismatic alkali amphibole from this vein gave a disturbed 40Ar/39Ar plateau age of 820 Ma. The disturbance suggests that the age should be considered a minimum age of vein formation. The chemical composition of this magnesio-arfvedsonite has nearly equal amounts of FeO and MgO (14.15 and 14.95 wt. percent, respectively), 1.95 wt. percent K2O, and 2.03 wt. percent F.
Based on textural evidence and abundance, alkali amphibole of hydrothermal metasomatic origin occurs in marble-hosted disseminated Fe ore samples 8B42; 796-48,252; 796-48,366; 8B7; and 796-48,277. The disseminated ores contain relicts of the marble host, small amounts of monazite or bastnaesite, and abundant alkali amphibole and magnetite. Apatite occurs locally. Chemically, these alkali amphiboles are generally slightly richer in FeO than MgO and contain from 1.77 to 3.28 wt. percent CaO. Alkali amphibole from sample 8B42 contains 2.82 wt. percent MnO (table 15), whereas alkali amphiboles from samples 796-48,366 and 796-48,277 are uniquely high in MnO (4.91 and 5.68 wt. percent, respectively, table 15). Alkali amphibole from sample 8B42 has a K-Ar age of 442+-11 Ma, determined by the Tianjin laboratory. The 40Ar/39Ar plateau ages of alkali amphibole from samples 796-48,252, 8B7, and 796-48,277, obtained by Conrad and McKee, are 419.5+-2.0, 395.5+-4.4, and 343.1+-3.0 Ma, respectively (table 15). The high MnO content of metasomatic alkali amphiboles generally seems to be characteristic of late-stage Fe mineralization. On the basis of textural relations for all these samples of marble-hosted disseminated ore, alkali amphiboles replaced small amounts of monazite, and the amphibole, in turn, was replaced by magnetite. Therefore, the magnetite mineralization may be younger than the youngest (343.1+-3.0 Ma) metasomatic alkali amphiboles (sample 796-48,277).
The ages of metasomatic alkali amphiboles correspond to episodes of intense hydrothermal activity in the Bayan Obo mine area, when most of the REE mineralization of banded ores took place. However, because the episodes are represented by different mineral assemblages, the mineralizing fluids for the metasomatic amphiboles and banded ores may have been unrelated. Many of the banded ores having high RE2O3 contents do not contain alkali amphiboles. As previously mentioned, where REE minerals coexist with alkali amphiboles in marble-hosted disseminated ores, the monazite and bastnaesite are usually much older than the alkali amphiboles, for example, in samples 8B7 and 796-48,277 (tables 14 and 15). Hornblende from a plagioclase-hornblende vein that crosscuts banded ore has a 40Ar/39Ar plateau age of 355.1+-4.8 Ma, which together with the age of alkali amphibole from a vein that crosscuts banded ore (sample 7B30-30), indicates that all banded ores in the Main and the East Orebodies were formed before about 396 Ma, the metamorphic age of the older amphibole.
Chao and others (1993) determined that remobilization of monazite was unlikely because any metal-transporting acid hydrothermal solutions entering along microfractures and grain boundaries within the carbonate host rock would tend to dissolve and remove the carbonate host, become neutralized, and precipitate the metals and other mineral components from such solutions. To a very minor extent, bands of fluorite in banded ores do show slight resorption of early minerals such as REE, Fe oxide, and gangue minerals, but resorption is limited to within the band itself. Therefore, remobilization and redistribution of early REE minerals and alkali amphibole are likely to have been minimal or insignificant.
The effect of metamorphism on mineral ages was estimated by Chao and others (1992, 1993). Metamorphism did not cause resetting of the ages of various generations of REE minerals because closure temperatures of monazite are higher than the temperatures reached during the low-grade Caledonian and Hercynian thermal events. Harland and others (1990) stated that the U-Th-Pb system closure temperature for monazite is in the range 600-500°C for cooling rates of 10-1°C/m.y., and Parrish (1990) cited a 725+-25°C closure temperature for the U-Pb system in monazite. The 232Th/208Pb internal isochrons of monazites and bastnaesites (figs. 18-22, 31A, and 32A; Wang and others, 1994) indicate no resetting of mineral ages.
According to Harrison (1981), the closure temperatures of hornblende are 578-490°C for cooling rates in the range 500-5°C/m.y. The ages of alkali amphiboles from samples Drew 1 and 7B13C have been slightly disturbed, probably by the latest Caledonian events. Conrad and McKee (1992) found a 10-percent decrease of the age of Drew 1 due to 40Ar loss. Multiple analyses of alkali amphibole subsamples from a vein (sample 7B13C, table 15) have disturbed 40Ar/39Ar spectra, the apparent ages ranging from about 820 to 640 Ma (fig. 33). The age of 820 Ma is considered the minimum age of this alkali amphibole. Because of evidence of disturbance of pre-Caledonian alkali amphiboles due to 40Ar loss, mineral ages of various generations of alkali amphiboles are considered minimum ages. However, the Caledonian ages for alkali amphiboles of various origins generally show good to excellent 40Ar/39Ar plateau release patterns (figs. 31B and 32B; Chao and others, 1993). Because of the good to excellent plateau release patterns of Caledonian-age alkali amphiboles, these alkali amphiboles do not appear to have been seriously affected by thermal events associated with the intrusion of granitic rocks or regional metamorphic events of Hercynian age.
In comparison with hornblende, biotite has a much lower estimated closure temperature, about 345-280°C for cooling rates in the range 100-1°C/m.y. (Harrison and others, 1985). The biotite in biotite and albitized-biotite schists of the H9 lithologic unit has been reset and has 40Ar/39Ar single-crystal total-fusion ages ranging from 309.8+-5.8 Ma to 297.1+-5.2 Ma. It is likely that the Hercynian granitic intrusion and regional metamorphic events have reset all biotite ages in the Bayan Obo mine region.
The key evidence for the epigenetic origin of the Bayan Obo ore deposit, in addition to the mineral textural relations, lies in the precise ages of episodes of mineralization of various generations of monazites and bastnaesites, and the minimum mineral ages of alkali amphiboles (fig. 33). The more than 100 carefully chosen doubly polished thin sections that were studied in detail provided the textural relations of both gangue and ore minerals replacing not only the host rocks but also ore and gangue minerals in various assemblages of disseminated and banded ores. A few of the selected replacement textures are shown in figures 14-17. Such textural data are corroborated with chemical compositions and isotopic mineral ages of the various generations of both alkali amphiboles and REE minerals. The resulting generalized mineral paragenetic sequence, updated from Chao and others (1993), is given in table 13.
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